Its really interesting to observe how the 3DS has been performing poorly given its a relatively new device with the purpose to replace the DS. Here is my personal take on why:
1. The Name. I think some people hear the name, "3DS" and think, 'ehh It sounds gimmicky, and that its only advantage from the DS is that it has 3D". I think many consumers think its just a DS with 3D and no real graphical improvements. So they pass on it. This has been the impressions of many casual/hardcore gamers I have talked to.
2. 3D Not Fully Adopted. 3D technology is still not a mainstream adoption. When you go to the movie theatres it seems the standard screens are much more filled with people than the 3D screens for many movies. I think many people can't justify the increase price of 3D yet. This also translates into Television sales. 3D TVs are not selling in huge numbers. Its mostly because of cost to desire ratio, and the need to buy the 3D glasses. Most people right now dont see the benefits of 3D TVs for the cost associated with them.
3. Smart Phones, Tablets. There is a growing adoption into smart phone, and tablet gaming. Whether we see it or not the Smart phone/Tablet gaming is taking away from the traditional gaming handhelds. This is going to increase, and many people still see gaming on their smart phone more attractive and easier than gaming on a 3DS.
So, I think 3DS is always going to have a hard time due to these factors. I do, however think the price drop will help, but not for very long. The Wii price drop seemed to help for maybe a couple months. I think a 3DS price drop may help sales for 4 to 6 months. But thats just my thoughts.